Drugs with narrow therapeutic indexes carry the disadvantage that small changes in the dosage can cause toxic effects. Patients taking such drugs often require extensive monitoring so that the level of medication can be adjusted to assure uniform and safe results. There is an imperative demand for doing away with the need for such monitoring without jeopardizing uniform and safe results. This need becomes particularly pronounced when such drugs are used in combination therapy.
Combination therapy, that is simultaneous administration of two or more medications to treat a single disease, is used in the treatment of a great number of conditions, such as for instance tuberculosis, leprosy, cancer, malaria, and HIV/AIDS.
In many instances, combination therapy carries a number of advantages, such as lower treatment failure rate, lower case-fatality ratios, slower development of resistance—and also less need for development of new drugs. However, combination therapy also suffers from a number of potential disadvantages, such as antagonism between the combined drugs, leading to reduction of their individual activity; increased risk of detrimental drug-drug interactions; increased cost compared with monotherapy; and increase in the potential for drug-related toxicity.
Combination chemotherapy is used extensively in the treatment of different types of cancer as it improves the survival profile compared to monotherapy. A better outcome can be obtained by combining antineoplastic agents with different mechanism of action that provides synergistic or additive effects during therapy.
The main problem with combination chemotherapy is the increase of toxicity compared with sequential administration of single agents. This may not be counterbalanced by limiting the doses, as this would lead to doses below the therapeutic ones. Different agents also have different pharmacokinetic properties and this makes it difficult to maintain the concentrations of all of them at optimal synergistic levels in the target tissues.
Doxorubicin is a drug commonly used in combination chemotherapy. It has an excellent antitumor activity, but unfortunately also a relatively low therapeutic index, as well as a broad spectrum of side effects which limits the use of it. One such side effect is cardiotoxicity, which is usually developed as an acute or subacute syndrome. At greater drug exposure myocardial damage may occur, and it is recommended that the cumulative lifetime dose of doxorubicin does not exceed 450-550 mg/m2. This cardiotoxicity is the main cause for preventing doxorubicin to be used in elderly patients and in patients with pre-existing cardiac disease.
Several attempts have been made to provide anthracyclines/taxanes combinations, for instance of doxorubicin and docetaxel, using conventional formulations. These compounds demonstrate high monotherapy activity with different mechanisms of action: doxorubicin inhibits the progression of the enzyme topoisimerase II throughout the whole cell cycle while docetaxel hinders a mitotic cell division between metaphase and anaphase by preventing physiological microtubule depolymerisation/disassembly. The provided combinations have shown to be more effective than the corresponding monotherapies, but also involving higher toxicity. This precludes the therapy from becoming a standard method for treatment of different types of cancer, especially in case of certain conditions such as febrile neutropenia.